drm/amd/display: dsc engine not disabled after unplug dsc mst hub

[WHY]
If timing and bpp of displays on mst hub are not changed,
pbn, slot_num for displays should not be changed. Linux
user mode may initiate atomic_check with different display
configuration after set mode finished. This will call to
amdgpu_dm to re-compute payload, slot_num of displays and
saved to dm_connect_state. stream->timing.flags.dsc, pbn,
slot_num are updated to values which may be different from
that were used for set mode. when dsc hub with 3 4k@60hz dp
connected, 3 dsc engines are enabled. timing.flags.dsc = 1.
timing.flags.dsc are changed to 0 due to atomic check. when
dsc hub is unplugged, amdgpu driver check timing.flags.dsc
for last mode set and find out flags.dsc = 0, then does not
disable dsc.

[HOW]
check status of  displays on dsc mst hubs. re-compute pbn,
slot_num, timing.flags.dsc only if there is mode, connect
or enable/disable change.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Hersen Wu <hersenwu@amd.com>
Reviewed-by: Mikita Lipski <Mikita.Lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>